Serious Mental Illness (SMI) is a stigmatized and disabling health condition that reduces average life expectancy by 25 years. SMI is also a prevalent public health problem affecting 25% of jail populations. Re- entry to the community following incarceration is a vulnerable time for justice-involved individuals with SMI, and SMI requires prompt and ongoing access to mental health and other healthcare services. Justice- involved individuals with SMI re-entering the community experience multiple barriers to access to community mental health, medical care and social services due to their debilitating symptoms, practical challenges accessing community services, and mental health stigma. Peer navigation has been found to improve access to the mental health and medical care among individuals with SMI in the community. However, no peer support interventions for SMI have been tested to assist with mental health service linkage during re-entry to the community after incarceration. This study will develop and pilot test a peer navigator intervention for individuals with SMI re-entering the community after jail stay, providing formative work for a larger randomized controlled trial evaluating the effectiveness of peer navigator intervention for justice-involved individuals with SMI. The intervention is based on social support theory. The project will: (a) develop a peer navigation intervention and evaluate its feasibility, acceptability, and potential engagement of target mechanisms for enrollment in mental health, medical care and substance use services among individuals with SMI re-entering the community and (b) conduct a randomized pilot trial in a sample of 40 individuals with SMI re-entering the community. Proposed target mechanisms include increased instrumental, informational, and emotional support for treatment engagement and recovery, as well as increased perception of social norms promoting treatment engagement and recovery. The control condition will be Standard Of Care (SOC). In addition to feasibility and acceptability, other outcomes include: (1) health service outcomes (primary) including enrollment/engagement/utilization of community mental health (primary), medical care and substance use services; and shorter days between release and first contact with healthcare provider; (2) Clinical outcomes: reduced psychiatric symptoms, increased functioning, adherence to psychiatric medications, fewer substance using days, fewer hospitalizations and suicide attempts; (3) Life context outcomes: nights unstably housed, and time until rearrest and (4) Potential target mechanisms that include instrumental, informational, and emotional support for treatment engagement and social norms about treatment engagement and recovery. Addressing the needs of re-entering individuals with SMI is a pressing priority for both the mental health and criminal justice systems. Peer navigators could play a critical role in continuing recovery and successful reintegration, reducing the impact of criminal justice involvement and mental health challenges.